Two basic requirements are made of such systems for fittings which are relevant to safety and which are installed, for example, in a power plant or in the chemical industry or in gas production or oil production facilities: in a normal mode the fitting must be continuously (or quasi-continuously) adjustable in order, for example, to regulate large steam flows. In an emergency mode it must be possible to adjust the fitting into a specific (for example closed) safety position even in the event of a failure of the system.
Document WO 2012/062404 discloses such a safety and movement control system for a fitting, in which system the two functions mentioned above are performed by separate hydraulic devices. An actuating cylinder of the fitting is supplied with pressure medium and set by means of a control arrangement in the normal mode. In an emergency, the actuating cylinder is adjusted into the desired position by means of a safety circuit. The safety circuit has seat valves connected in series in its main lines, by means of which the chambers of the actuating cylinder can be connected to high pressure and to low pressure. The control arrangement has a proportional valve for supplying pressure medium to the actuating cylinder in a way which can be metered finely.
A disadvantage with such safety and movement control systems and of other known from the prior art, is that the proportional valve, or even servo valves which are used have internal leakage. As a result, the most frequent causes of failure of these systems are gap filtration and silting. In the case of gap filtration, very small particles which cannot be filtered out of the oil with an acceptable degree of expenditure cause the control piston of the proportional valve to stick. In the case of silting, the gaps which do not have any flow speeds, or only low flow speeds, are made continuously more narrow by submicron oil components. This can also cause the proportional valve to fail and contributes, in particular, to the sudden failure as a result of gap filtration.
Furthermore, the leakage of the proportional valve makes it necessary to have a seat valve which is connected in series therewith and which serves as a blocking element in the working line. By closing the seat valve, the actuating cylinder and therefore the main valve or the fitting can be held without leakage in precisely the adopted or selected position.